ALUMINIUM AND ITS ALLOYS 271 IMITATION PLATE Imitation plate for table ware, trays, etc., consists of argentan with a low proportion of nickel (7-10%) heavily silvered galvanically (it contains 2-3% Ag). Besides the trade-mark, it often exhibits—particularly with forks, spoons, etc.—a number indicating the quantity of silver deposited per dozen pieces. The alloy is analysed like argentan. Usually, however, it suffices to determine only the layer of silver and this may be effected as follows : Determination of the Silver.—i. After being well cleaned and freed from grease, the object is suspended by a platinum wire in a 2-3% potassium cyanide solution in a tall, narrow cylinder and is connected with the positive pole of a current source. A thin, clean copper sheet in communication with the negative pole, is also suspended in the liquid but not in contact with the object. The current (o-1-0-2 ampere) dissolves the silver from the article and deposits it on the copper. When the de-silvering is com- plete, both the object and the silvered copper are removed and washed, the latter being dissolved in nitric acid, the solution diluted and the silver precipitated by a slight excess of hydrochloric acid; the silver chloride is collected in a Gooch crucible, washed, dried and weighed. The hydrocyanic solution is acidified with dilute hydrochloric acid (under a hood], the liquid evaporated until cyanogen compounds are com- pletely eliminated, and the precipitated silver chloride weighed. From the sum of the two quantities of silver chloride the amount of silver on the object is calculated. 2. The article, or part of it, is freed from grease, weighed, and gently heated with a mixture of 9 vols. of cone, sulphuric acid and I vol. of cone, nitric acid. By this means all the surface silver is rapidly dissolved, whilst the metal beneath is not at all or but little attacked. When the de-silvering is complete, the object is withdrawn, washed rapidly and thoroughly with water,1 dried and weighed. The loss in weight gives the silver plating. For a more rigorous determination the silver dissolved may be esti- mated by diluting with water the nitric-sulphuric solution, together with the washing water, and determining the silver either volumetrically by Volhard's method or gravimetrically as chloride. ALUMINIUM AND ITS ALLOYS Owing to its lightness and stability, aluminium is now used for making many diverse objects in common use and for naval and flying construction. Further, aluminium forms a constituent of numerous alloys, many of which are mechanically superior to pure aluminium. Among these are : Light aluminium-bronze (Al with 3-8% Cu) ; Magnalium (Al with 3-15% Mg) ; Barbouze's alloy (Al with 10% Sn) ; Z-iskon (Al with varying proportions of zinc) ; aluminiiim-nickel (Al with 1-3% Ni) ) aluminium-manganese 1 Thorough washing is effected by taking the object quickly from the acid mixture and immersing it in a fairly large vessel full of water.ric Analysis of Ordinary Brasses.